The present invention relates to a circuit for generating an output voltage which is proportional to temperature with a required gradient.
Such circuits exist which rely on the principle that the difference in the base emitter voltage of two bipolar transistors with differing areas, if appropriately connected, can result in a current which has a positive temperature coefficient, that is a current which varies linearly with temperature such that as the temperature increases the current increases. This current, referred to herein as Iptat, can be used to generate a voltage proportional to absolute temperature, Vptat, when supplied across a resistor.
One such practical difficulty is the need to accurately control the required gradient of variation of the voltage with respect to temperature. In a circuit of the type mentioned above, this can be done by controlling the value of resistance through which the current proportional to absolute temperature Iptat is supplied. However, this may not give adequate control of the gradient and it is desirable therefore to incorporate a second stage which allows the finer adjustment of the gradient to be made. It is an aim of the present invention to incorporate such a second stage in an environment with good line regulation for the first and second stages.
The present invention provides a circuit for generating an output voltage proportional to temperature with a required gradient, the circuit comprising: a first stage arranged to generate a first voltage which is proportional to temperature with a predetermined gradient, the first stage comprising: first and second bipolar transistors with different emitter areas having their emitters connected together and their bases connected across a bridge resistive element, wherein the collectors of the transistors are connected to an internal supply line via respective matched resistive elements such that the voltage across the bridge resistive element is proportional to temperature; a differential amplifier having its inputs connected respectively to said collectors, and its output connected to stabilisation circuitry connected between first and second power supply rails and an internal supply line which cooperates with the differential amplifier to maintain a stable voltage on the internal supply line despite variations between the first and second power supply rails; and a second stage which comprises a gain circuit connected to receive the first voltage for altering the predetermined gradient to match the required gradient, the gain circuit having as its voltage supply said stable voltage on the internal supply line.